1. Field of the Invention
The invention relates to an objective designed as a microlithography projection objective. The objective according to the invention comprises at least one liquid lens made from a transparent liquid.
2. Description of the Related Art
Microlithography projection objectives of multivarious design are known.
In all imaging systems, the smallest resolvable structural width is proportional to the Numerical aperture NA at the image plane.
This, in turn, is proportional to the angle of incidence and the refractive index nI of the medium through which the light falls onto the image plane.
By contrast with so-called dry objectives with gas (air, N2, He and the like) or a vacuum with a refractive index of approximately 1.0, a material, in particular a liquid, with a substantially higher refractive index is used as this medium in immersion systems.
For example, as far as is known for the wavelength 193 nm water has a refractive index nH2O=1.44.
High-index lenses with a refractive index much higher than 1.6 have been used in microlithography at wavelengths of greater than 365 nm, but they become incapable of use at the wavelengths of practical relevance such as 248 nm, 195 nm, 157 nm, since they are not sufficiently transparent, and so on. Lenses made from sapphire have a high refractive index but are birefringent, and this must be compensated in a complicated way and with limited success.